A student dissolves copper sulfate crystals in water and then wants to recover pure copper sulfate crystals from the solution. Describe the process of crystallisation that the student should use to obtain dry copper sulfate crystals from the solution.

Edexcel GCSE Chemistry (1CH0) — 2.2 Methods of separating and purifying substances · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Heat the copper sulfate solution until it becomes saturated and crystals start to form.
Allow the saturated solution to cool slowly so that crystals grow.
Once the crystals have formed, filter the solution to separate the crystals from the remaining liquid.
Dry the filtered crystals to obtain pure copper sulfate.

Examiner tips

  • Use the exact sequence: heat, saturate, cool, filter, dry. Mention that cooling allows crystal growth. Show understanding of why filtration separates solids from liquid. Include that the crystals should be dried to remove water.
  • common_mistakes
  • :
  • Not stating that the solution must be saturated before cooling. Failing to mention filtration as the separation step. Omitting the drying step, leaving crystals wet.

Mark scheme (4 marks)

  1. Heat the copper sulfate solution
  2. Heat until crystals start to form (saturated solution reached)
  3. Leave the solution to cool so that crystals form
  4. Filter the crystals from the remaining solution to collect them

Key terms in this question

crystallisation · solution

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